1.0 Field of Invention
The present invention generally relates to organic Rankine cycle working fluids. More particularly, the invention relates to chloro- and bromo-fluoro-olefins as organic Rankine cycle working fluids.
2.0 Description of Related Art
Water, usually in the form of steam, is by far the most commonly employed working fluid used to convert thermal energy into mechanical energy. This is, in part, due to its wide availability, low cost, thermal stability, nontoxic nature, and wide potential working range. However, other fluids such as ammonia have been utilized in certain applications, such as in Ocean Thermal Energy Conversion (OTEC) systems. In some instances, fluids such as CFC-113 have been utilized to recover energy from waste heat, such as exhausts from gas turbines. Another possibility employs two working fluids, such as water for a high temperature/pressure first stage and a more volatile fluid for a cooler second stage. These hybrid power systems (also commonly referred to as binary power systems) can be more efficient than employing only water and/or steam.
To achieve a secure and reliable power source, data centers, military installations, government buildings, and hotels, for example, use distributed power generation systems. To avoid loss of service that can occur with loss of grid power, including extensive cascading power outages that can occur when equipment designed to prevent such an occurrence fails, the use of distributed power generation is likely to grow. Typically, an on-site prime mover, such as a gas microturbine, drives an electric generator and manufactures electricity for on-site use. The system is connected to the grid or can run independent of the grid in some circumstances. Similarly, internal combustion engines capable of running on different fuel sources are used in distributed power generation. Fuel cells are also being commercialized for distributed power generation. Waste heat from these sources as well as waste heat from industrial operations, landfill flares, and heat from solar and geothermal sources can be used for thermal energy conversion. For cases where low- to medium-grade thermal energy is available, typically, an organic working fluid is used in a Rankine cycle (instead of water). The use of an organic working fluid is largely due to the high volumes (large equipment sizes) that would need to be accommodated if water were used as the working fluid at these low temperatures.
The greater the difference between source and sink temperatures, the higher the organic Rankine cycle thermodynamic efficiency. It follows that organic Rankine cycle system efficiency is influenced by the ability to match a working fluid to the source temperature. The closer the evaporating temperature of the working fluid is to the source temperature, the higher the efficiency will be. The higher the working fluid critical temperature, the higher the efficiency that can be attained. However, there are also practical considerations for thermal stability, flammability, and materials compatibility that bear on the selection of a working fluid. For instance, to access high temperature waste heat sources, toluene is often used as a working fluid. However, toluene is flammable and has toxicological concerns. In the temperature range of 175° F. to 500° F. (79° C. to 260° C.), non-flammable fluids such as HCFC-123 (1,1-dichloro-2,2,2-trifluoroethane) and HFC-245fa (1,1,1,3,3-pentafluoropropane) are used. However, HCFC-123 has a relatively low permissible exposure level and is known to form toxic HCFC-133a at temperatures below 300° F. To avoid thermal decomposition, HCFC-123 may be limited to an evaporating temperature of 200° F.-250° F. (93° C.-121° C.). This limits the cycle efficiency and work output. In the case of HFC-245fa, the critical temperature is lower than optimum. Unless more robust equipment is used to employ a trans-critical cycle, the HFC-245fa organic Rankine cycle is held below the 309° F. (154° C.) critical temperature. To increase the useful work output and/or efficiency of an organic Rankine cycle beyond the limitations noted above for HCFC-123 and HFC-245fa, it becomes necessary to find working fluids with higher critical temperatures so that available source temperatures such as gas turbine and internal combustion engine exhaust can be approached more closely.
Certain members of a class of chemicals known as HFCs (hydrofluorocarbons) have been investigated as substitutes for compounds known as CFCs (chlorofluorocarbons) and HCFCs (hydrochlorofluorocarbons). Yet both CFCs and HCFCs have been shown to be deleterious to the planet's atmospheric ozone layer. The initial thrust of the HFC development was to produce nonflammable, non-toxic, stable compounds that could be used in air conditioning/heat pump/insulating applications. However, few of these HFCs have boiling points much above room temperature. As mentioned above, working fluids with critical temperatures higher than, for example, HFC-245fa, are desirable. Since boiling point parallels critical temperature, it follows that fluids with higher boiling points than HFC-245fa are desired.
A feature of certain hydrofluoropropanes, including HFC-245fa as compared to fluoroethanes and fluoromethanes, is a higher heat capacity due, in part, to an increase in the vibrational component contribution. Essentially, the longer chain length contributes to the freedom to vibrate; noting, of course, that the constituents and their relative location on the molecule also influence the vibrational component. Higher heat capacity contributes to higher cycle efficiency due to an increased work extraction component and also an increase in overall system efficiency due to improved thermal energy utilization (higher percentage of the available thermal energy is accessed in sensible heating). Moreover, the smaller the ratio of latent heat of vaporization to heat capacity, the less likely there will be any significant pinch point effects in heat exchanger performance. Hence, in comparison to HFC-245fa and HCFC-123, working fluids that possess, for example, higher vapor heat capacity, higher liquid heat capacity, lower latent heat-to-heat capacity ratio, higher critical temperature, and higher thermal stability, lower ozone depletion potential, lower global warming potential, non-flammability, and/or desirable toxicological properties would represent improvements over fluids such as HFC-245fa and HCFC-123.
Industry is continually seeking new fluorocarbon based working fluids which offer alternatives for refrigeration, heat pump, foam blowing agent and energy generation applications. Currently, of particular interest, are fluorocarbon-based compounds which are considered to be environmentally safe substitutes for fully and partially halogenated fluorocarbons (CFCs and HCFCs) such as trichlorofluoromethane (CFC-11), 1,1-dichloro-1-fluoroethane (HCFC-141b) and 1,1-dichloro-2,2-trifluoroethane (HCFC-123) which are regulated in connection with the need to conserve the earth's protective ozone layer. Similarly, fluids that have a low global warming potential (affecting global warming via direct emissions) or low life cycle climate change potential (LCCP), a system view of global warming impact, are desirable. In the latter case, organic Rankine cycle improves the LCCP of many fossil fuel driven power generation systems. With improved overall thermal efficiency, these systems that incorporate organic Rankine cycle can gain additional work or electric power output to meet growing demand without consuming additional fossil fuel and without generating additional carbon dioxide emissions. For a fixed electric power demand, a smaller primary generating system with the organic Rankine cycle system incorporated can be used. Here, too, the fossil fuel consumed and subsequent carbon dioxide emissions will be less compared to a primary system sized to supply the same fixed electric power demand. The substitute materials should also possess chemical stability, thermal stability, low toxicity, non-flammability, and efficiency in-use, while at the same time not posing a risk to the planet's atmosphere. Furthermore, the ideal substitute should not require major engineering changes to conventional technology currently used. It should also be compatible with commonly used and/or available materials of construction.
Rankine cycle systems are known to be a simple and reliable means to convert heat energy into mechanical shaft power. Organic working fluids are useful in place of water/steam when low-grade thermal energy is encountered. Water/steam systems operating with low-grade thermal energy (typically 400° F. and lower) will have associated high volumes and low pressures. To keep system size small and efficiency high, organic working fluids with boiling points near room temperature are employed. Such fluids have higher gas densities lending to higher capacity and favorable transport and heat transfer properties lending to higher efficiency as compared to water at low operating temperatures.
In industrial settings, there are more opportunities to use flammable working fluids such as toluene and pentane, particularly when the industrial setting has large quantities of flammables already on site in processes or storage. For instances where the risk associated with use of a flammable working fluid is not acceptable, such as power generation in populous areas or near buildings, non-flammable fluorocarbon fluids such as CFC-11, CFC-113 and HCFC-123 are used. Although these materials are non-flammable, they were a risk to the environment because of their ozone-depletion potential.
Ideally, the organic working fluid should be environmentally acceptable, that is, have little or no ozone depletion potential and low global warming potential, non-flammable, of a low order of toxicity, and operate at positive pressures. More recently, hydrofluorocarbons such as HFC-245fa, HFC-365mfc, and HFC-43-10mee have been employed as organic Rankine cycle working fluids either neat or in mixtures with other compounds. With regard to global warming potential of working fluids, existing fluids based on hydrofluorocarbons such as HFC-245fa, HFC-356mfc, HFC-43-10, hydrofluoroethers such as commercially available HFE-7100 (3M) have global warming potentials that may be considered unacceptably high in light of a given countries environmental circumstances and subsequent regulatory policies.
Organic Rankine cycle systems are often used to recover waste heat from industrial processes. In combined heat and power (cogeneration) applications, waste heat from combustion of fuel used to drive the prime mover of a generator set is recovered and used to make hot water for building heat, for example, or for supplying heat to operate an absorption chiller to provide cooling. In some cases, the demand for hot water is small or does not exist. The most difficult case is when the thermal requirement is variable and load matching becomes difficult, confounding efficient operation of the combined heat and power system. In such an instance, it is more useful to convert the waste heat to shaft power by using an organic Rankine cycle system. The shaft power can be used to operate pumps, for example, or it may be used to generate electricity. By using this approach, the overall system efficiency is higher and fuel utilization is greater. Air emissions from fuel combustion can be decreased since more electric power can be generated for the same amount of fuel input.